Abundance analysis of a nitrogen-rich extreme-helium hot subdwarf from the SALT survey

Abstract

ABSTRACTWe have performed a detailed spectral analysis of the helium-rich hot subdwarf EC 20187−4939, using data obtained in the SALT survey of helium-rich hot subdwarfs. We have measured its effective temperature, surface gravity, and chemical abundances from the spectrum. Its radius has also been determined by fitting the spectral energy distribution using photometric data, from which a mass of 0.44$^{+0.32}_{-0.19}$ M⊙ has been inferred using the measurement of surface gravity. This star is particularly abundant in helium and nitrogen, whilst being both carbon and oxygen-weak. The surface abundances and mass have been found to be consistent with a helium white dwarf merger product. The abundance effects of alpha captures on nitrogen during the merger process and possible connections between EC 20187−4939, and other carbon-weak related objects are discussed.